Recently, in most of gasoline engines used in passenger cars, the fuel supply amount, ignition timing and the like are electronically controlled in view of pollution control of exhaust gas and in order to assure the engine performance such as output. The carburetor and throttle valve of an electronic fuel injection device have heretofore been opened and closed according to the intention of the driver through a cable directly connected to the acceleration pedal. However, with the advance of auto-cruising devices and mechanical automatic transmissions, there has been developed a DBW type throttle valve control apparatus which electronically controls the throttle valve by an electric actuator such as a servo motor.
In general, in this type of throttle valve control apparatus, various data such as the clutch connection condition and intake air temperature, in addition to the pressing-down amount of the acceleration pedal, are processed by a microprocessor to control the throttle valve to an optimum opening according to the current running condition of the vehicle. As a result, while an auto-cruising device is in operation, for example, the throttle valve opening can be automatically controlled so that the vehicle speed is maintained at a preset value regardless of the load conditions such as running on an ascending slope, or, in a clutch-off state in a mechanical automatic transmission, the throttle valve can be easily closed independently of the operation amount of the acceleration pedal.
However, in a DBW type throttle valve control apparatus, there may a case in which the throttle valve opening becomes uncontrollable due to a malfunction of its control system including the motor.
To cope with such a case, there is a method in which the throttle valve is provided with a return spring to urge the throttle valve to the closing direction, and, when a malfunction occurs, the throttle valve is fully closed by the force of the spring. However, with this method, there may be a case in which desired output characteristics cannot be obtained when a malfunction occurs, resulting in an impaired running stability of the vehicle.